Demand for high-output power semiconductor parts, which are for use in an inverter of a hybrid car or the like, is rapidly increasing in recent years.
In a power semiconductor part as described above, a semiconductor is generally bonded with a ceramic substrate. Examples of the ceramic substrate include a laminated substrate as copper-metalized alumina ceramics, i.e., direct bonded copper (DBC) substrate, and a laminated substrate as aluminum-metalized aluminum nitride, i.e., direct bonded aluminum (DBA) substrate.
Since it is preferable that a semiconductor quickly dissipates heat generated by itself, the semiconductor is cooled by air or water eventually. To efficiently carry out such cooling by air or water and prevent the cooling from adversely affecting other elements in a relatively small space in an electronic device, a ceramic substrate generally bonded with a semiconductor by solder is further combined with a material having relatively high thermal conductivity to constitute a laminated structure.
A ceramic substrate, having relatively low thermal expansion coefficient like a semiconductor material such as Si, can achieve reliable bonding between a semiconductor and itself.
The most commonly used heat sink material to be laminated with such a ceramic substrate as described above is pure copper. That is, a ceramic substrate is often air-cooled by way of copper plate laminated therewith. However, there is a problem with pure copper that pure copper exhibits relatively high thermal expansion coefficient and thus relatively large discrepancy in thermal expansion coefficient between the ceramic substrate and itself, thereby generating relatively large thermal stress at the bonding interface between the copper plate and the ceramic substrate and causing damage to the semiconductor and/or causing the semiconductor to come off from the ceramic substrate. Accordingly, in a case where such a conventional heat sink material as described above is used, the heat sink material must have sufficient thickness to alleviate thermal stress.
An inverter for use in a hybrid car, which generates a relatively large amount of heat, is coupled with a cooling device made of aluminum for water cooling. Examples of a heat sink material in this case include a Mo—Cu material having relatively high thermal conductivity and relatively low thermal expansion coefficient. However, such a Mo—Cu material as described above as a heat sink material, although it achieves reliable bonding between a ceramic substrate and itself due to relatively close thermal expansion coefficients thereof, causes a problem that the Mo—Cu material and the cooling device made of aluminum have to be coupled with each other by screws by way of silicon grease because of relatively large difference in thermal expansion coefficient between the Mo—Cu material and the cooling device made of Al.
We disclosed in JP-B 4138844, as an alternative material for the Mo—Cu material described above, a relatively inexpensive Cr—Cu alloy having both relatively low thermal expansion properties and relatively high thermal conductivity. The Cr—Cu alloy described above is produced by making Cu infiltrate a porous Cr sintered body and then subjecting the sintered body to rolling. As shown in the Example of JP-B 4138844, a rolled material (rolling reduction rate: 72%) exhibits a good effect of decreasing thermal expansion coefficient. However, in this case, when thermal conductivity of the material in an in-plane direction thereof is compared with thermal conductivity in thickness direction, the former is approximately 200 W/mK, while the latter is approximately 150 W/mK, indicating relatively poor thermal conductivity in thickness direction of the material. When the Cu-infiltrated Cr sintered body is rolled, the Cr phase is expanded in the rolling direction and the resulting Cr—Cu alloy exhibits a laminated structure of Cr layers as shown in FIG. 1 of JP-B 4138844. This Cr phase (specifically, layered-grains of the Cr phase) having lower thermal conductivity than the Cu phase presumably disturbs smooth dissipation of heat in thickness direction of the rolled material, although the Cu phase somehow continuously exists in the thickness direction. In contrast, it is assumed that heat can easily dissipate along the Cu phase in an in-plane direction of the rolled material. It has been confirmed that the larger rolling reduction rate results in the larger difference in characteristics between an in-plane direction and the thickness direction of the rolled material.
In a case where the aforementioned Cr—Cu alloy is used for a heat sink plate or sheet, the problem of directionality of thermal conductivity caused by rolling is rendered relatively insignificant when heat is generated from only a part of the relevant surface because then a good effect of dissipating heat in in-plane directions can be expected in a compensating manner in spite of relatively low thermal conductivity in the thickness direction. However, when heat is received by the entire portion of the relevant surface of the heat sink plate and has to be dissipated from another or the opposite surface of the heat sink plate, the heat sink plate cannot demonstrate satisfactory heat-sink properties because thermal conductivity in the thickness direction of the heat sink plate then substantially determines the heat-sink properties of the heat sink plate.
JP-A 2007-035985 discloses a Cr—Cu/Cu composite alloy having one surface constituted of Cr—Cu alloy and the other surface constituted of Cu. That composite alloy is produced by placing a Cu plate on a porous Cr sintered body and making a portion of Cu infiltrate the Cr sintered body by a heating treatment so that the rest of Cu remains on the Cr sintered body, thereby obtaining a two-layer structure including a Cr—Cu alloy layer and a Cu layer.
In the case of such a composite alloy as described above, however, there is a problem that shrinkage cavity is generated in the Cu layer due to solidification shrinkage after the infiltration treatment. The larger size of the sintered body leads to the more conspicuous shrinkage cavity, which is hard to handle. Thickness of a Cu layer must be increased to form the Cu layer free of such shrinkage cavity as described above. However, increase in thickness of a Cu layer raises a concern of higher cost for materials.
It could therefore be helpful to provide a heat sink for an electronic device having relatively low thermal expansion properties, excellent thermal conductivity particularly in the direction of thickness, and a reduced entire thickness, as well as an advantageous method for producing the heat sink.